The Milky Way Flipped its Disk Billions of Years Ago
The Milky Way is an ancient structure that carries the scars of a violent history. Astronomers have long observed that our galaxy possesses two distinct disks. The primary spiral disk contains most stars, while a sparse, stellar halo—or thick disk—surrounds it. Recent data from the Gaia mission shows that this outer halo rotates significantly slower than the inner disk. Researchers from Durham University recently presented findings that explain this discrepancy through simulations of galaxy formation.
By examining twenty-five galaxy models similar to our own, the team found a clear pattern. Galaxies that mirror the Milky Way's current structure underwent a major collision, specifically one akin to the Gaia-Enceladus-Sausage merger. This violent interaction forced the galaxy to experience a disk-flip event billions of years ago. This event means that the majority of stars in our galaxy, including those in our own solar system, likely occupied different trajectories in the past compared to their current orbits.
Kirill Batrakov, the lead researcher, notes that living inside the Milky Way provides a unique advantage for study. We can observe individual stars with a level of precision impossible in more distant galaxies. This allows scientists to reconstruct the complex history of the galaxy from current observations. The simulations suggest this flip was a critical phase that helped determine the current, slow rotation of the outer halo.
These findings also provide insight into the dark matter halo surrounding our galaxy. Evidence indicates that the dark matter halo is twisted and aligned with the inner disk. This suggests the dark matter and stellar halo evolved in tandem as the galaxy absorbed smaller neighbors over time. These results add a major chapter to our understanding of the Milky Way, proving that our home is not as stable as it appears from our perspective on Earth.